Domain Name System Client Behavior in Windows Vista

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Microsoft® Windows Vista™ includes both Internet Protocol version 4 (IPv4) and Internet Protocol version 6 (IPv6) protocol stacks that are installed and enabled by default. Domain Name System (DNS) name queries and registrations can now involve both IPv4 address records (A records) and IPv6 address records (AAAA records). This article describes the behavior of the DNS Client service in Windows Vista for DNS queries and registrations and the possible impact on DNS traffic.

Note: This article does not describe changes to DNS client behavior in versions of Windows released after Windows Vista with Service Pack 1 and Windows Server 2008, including additional service packs or other updates.

DNS Query Behavior

Computers running Windows Vista need to perform both A and AAAA queries to determine the best method of connectivity to the desired endpoint. By obtaining both IPv4 and IPv6 addresses, there is an increased chance of being able to access the desired endpoint if one of the addresses is unreachable.

The DNS Client service in Windows Vista has been designed to minimize the impact on DNS servers when performing DNS name queries through the following behavior:

If the host has only link-local or Teredo IPv6 addresses assigned, the DNS Client service sends a single query for A records.

If the host has at least one IPv6 address assigned that is not a link-local or Teredo address, the DNS Client service sends a DNS query for A records and then a separate DNS query to the same DNS server for AAAA records. If an A record query times out or has an error (other than name not found), the corresponding AAAA record query is not sent.

This DNS querying behavior will assist enterprises and ISPs in their transition to IPv6. When AAAA records are added to DNS either manually or through DNS dynamic update, computers running Windows Vista will by default use IPv6 over IPv4, providing proof to IT staff that the IPv6 routing and name resolution infrastructure is working properly for IPv6 connectivity. When organizations transition to an IPv6-only infrastructure and disable IPv4, the DNS Client service will send only AAAA queries.

Computers running Windows Vista can increase DNS traffic and the load on DNS servers when the computers have been configured with a global address:

On intranets that have deployed IPv6 (either native or ISATAP), there will be additional DNS query traffic. However, deployments of Windows Vista in enterprise networks using IPv6 have not resulted in dramatic increases in the loads on intranet DNS servers. Intranets running at or near capacity for DNS might need to provide additional capacity to better support an enterprise deployment of Windows Vista.

On the Internet, computers running Windows Vista by default will typically not be configured with a global address from their ISPs or from their gateway devices. At this time, typical ISPs do not support native IPv6 connectivity and typical home routers do not support the 6to4 transition technology. If a computer running Windows Vista is located behind a network address translator (NAT), the Teredo client component will automatically configure a global Teredo address, even if it is in an inactive state. However, a computer running Windows Vista will not send AAAA record queries if it only has a Teredo address assigned.

DNS Registration Behavior

The DNS Client service in Windows Vista uses DNS dynamic update and attempts to register the following records:

A records for all IPv4 addresses assigned to the interfaces that are configured with a DNS server

Pointer (PTR) records for IPv4 addresses assigned to interfaces that are configured with a DNS server

AAAA records for all global IPv6 addresses assigned to interfaces that are configured with a DNS server

PTR records for IPv6 addresses assigned to interfaces that are configured with a DNS server

Teredo addresses are not registered.

For the typical intranet host configured with a global IPv6 address, additional AAAA records for IPv6 global addresses are registered. However, a DNS dynamic update client includes all of the records that they are registering in a single packet. Therefore, although there are additional records to register, there are no additional packets for AAAA record registration. Therefore, the impact on intranet DNS servers for AAAA registration is minimal.

Typical Internet-based DNS servers do not support DNS dynamic update. However, if an Internet-based DNS server did support DNS dynamic update, there is typically no additional traffic because the typical Windows Vista-based Internet host does not have a global IPv6 address assigned and Teredo addresses are not registered. Therefore, for typical Windows Vista-based hosts on today's Internet, there is no performance impact on Internet DNS servers for DNS registration.